Background
Basic considerations
Fundamentals of harness and implement design
Validation of the equation linking implement draught to weight and angle of pull
Significance of the tillage implement draught equation
Matching the animal, the harness and the implement
Bibliography
F. Inns
The author was formerly Professor of Agricultural Machinery Engineering at
Silsoe College, Cranfield University, Bedford, the United Kingdom, following
which he was awarded an Emeritus Fellowship by the Leverhulme Trust to undertake
studies on equipment design for animal draught tillage. His address is: 53 Alameda
Road, Ampthill MK45 2LA, United Kingdom, Tel: 0044 1525 402508, Fax: 0044 1525
406373.
Acknowledgements
The author Is grateful to the Leverhulme Trust for the award of the Emeritus Fellowship, which enabled him to undertake the work reported in this article; to the Director and staff of the Centre for Tropical Veterinary Medicine, Edinburgh, United Kingdom, particularly Dr Anne Pearson; to Tom Copland, Head of the Mechanisation Section, Scottish Centre of Agricultural Engineering, Midlothian, United Kingdom; to Dr M.A.M. Msabaha, Zonal Director of Research and Training, Southern Highlands Zone, Mbeya, the United Republic of Tanzania; to Richard Shetto, Head of Agricultural Engineering, Ministry of Agriculture Research and Training Centre, Mbeya; to numerous farmers and farm workers In the Mbeya District; and to all others who have been interested and/or involved in the investigations.
Tillage operations involve an interaction between an implement and the soil, Usually aimed at land and/or seed-bed preparation, weed control or assistance in soil moisture management.
When undertaken using human labour, these operations are generally associated with farming drudgery; they demand intensive power inputs but rarely require the judgement or control that helps to provide interest and mental stimulus for the operator. Animal power can increase the worker's motivation and status while enabling more land to be cultivated with reduced demands on human energy inputs.
Animal draught tillage is part of a complex system affected by a wide range of interacting factors at different levels, as indicated in Figure 1. At the fundamental (operator) level, the animal draught tillage system comprises the draught animal, the tillage implement, the harness, the operator and the soil to be cultivated. The fundamental system is influenced by the farming system of which it is a part and, beyond that, by the national infrastructure and- policies that can determine the availability of local skills and the cost of many essential inputs, often as the result of political decisions.
Any system works best when all factors are well matched in achieving their purpose. At present, many animals are overloaded. This article shows how it is possible, by a judicious mixture of theoretical and practical knowledge, to match draught animals with appropriate cultivation implements in order to make the best use of their work potential, while bearing in mind the constraints to which most small-scale farmers are subject.
Fundamental requirements of the animal draught tillage system are that:
· the implement must do the job the farmer wants changing the soil condition or eliminating weeds according to the need;· the implement must not overload the animals - they should tee able to manage as full season's work without stress;
· the harness must provide an efficient and safe connection between the animals and the implement with workloads distributed comfortably on the animals;
· the implement and harness should be easily adjustable to meet the above conditions and to ensure that the operator can control the system;
· the whole system must be user-friendly and sustainable in the context of the farmers' resources and the available infrastructural support.
These requirements are usually met in varying degrees by traditional systems that have been developed by observation and experiment over many centuries. Modern systems must be developed more quickly to meet the different circumstances and pressing needs of today's farmers. The necessary accelerated development depends on using both theoretical and practical knowledge to maximum effect.
Draught capability of work animals
The draught capability of a work animal is the force that it can exert to pull an implement. An animal's draught capability is limited by biomechanical, physiological and environmental factors, which are mainly a consequence of, one, certain features of the particular animal, such as animal species, conformation, health and nutritional status, and body mass of the animal, and, two, the operators' ability to use and manage their animals, such as working speed and duration of the effort (length of the working day) and attention given to the animal's welfare, including feed, comfort, training, health and handling.
As a general rule, provided all other factors are favourable, bovines (mainly cattle and buffaloes) should be able to provide a sustainable draught force of 10 to 12 percent of their body weight, while equines (mainly horses, donkeys and mules) and camels are able to sustain draughts of 12 to 14 percent of their body weight. Farmers and operators are rarely able to measure the draught force and must be sensitive to signs of physical discomfort and unwillingness to continue working, which are associated with overload stress. It is often possible to alleviate the overload promptly by suitable adjustments to the implement and harnessing there is nothing to be gained by forcing an animal to work beyond its innate capability.
Harness and implement types
Harness and implement designs have progressed on the basis of observation and experimentation over thousands of years, leading to some remarkably well-matched combinations, of which the two-ox shoulder yoke and beam implement shown in Figure 2 is a noteworthy example. More recent developments have taken a number of directions, many of which have been heavily promoted by interested parties but have not yet found acceptance. It is not possible to consider all of these in detail in the brief review made below.
Principal types of harnesses
The shoulder (neck or withers) yoke is one of the earliest harness types and is still the most widely used for a pair of bovines - it is not suitable for equines. Collar, split-collar (three-pad) and breastband harnesses have been developed for equines. However, when used with a team of two or more animals harnessed together, usually through special arrangements of swingletrees and eveners, their use becomes rather cumbersome. A range of harness designs is described in detail by Barwell and Ayre (1982), including experimental prototypes that have not been taken up by farmers.
Principal types of cultivation implements
There are two basic types of cultivation implement for animal draught- beam- and chain-pulled. Beam-pulled implements, such as that shown in Figure 2, trace their ancestry more or less directly from the first recorded animal draught implements used more than 4 000 years ago. Since then, development has led to many traditional designs that are extremely efficient and easy to control. Beam-pulled implements are the most commonly used type throughout Asia and in adjacent regions.
Chain-pulled implements are used extensively in Africa, south of the Sahara, and in Latin America, having been introduced from Europe and North America. Early European ploughs were very heavy and used various devices to support part of their weight. Some depended on wheels, while others, known as swing ploughs, were fitted with a soleplate or slade, which provided support by sliding along the furrow bottom.
Most swing ploughs currently manufactured in Africa and India are based on designs dating from the 1 930s. They are unnecessarily heavy - those shown in Figure 2 weigh about 35 to 40 kg - and are fitted with a small nosewheel towards the front of the beam to assist in turning and transport. The nosewheel is not intended to support the plough in work, attempts to use it for that purpose have led to excessive wear on bearings, inefficiency and to difficulties of adjustment and control.
Modern swing ploughs can be made lighter, stronger and more cheaply by using higher grade steel, improved design and welded construction. The weight of the plough shown in Figure 2, with a plough body of equal size to the 1 930s type, is less than 18 kg. The lighter ploughs do not need a nosewheel; when correctly designed and used with a suitable harness, they are easy to adjust and turn in work and easy to transport.
Animals, harnesses and implements have generally been treated by researchers as separate components of the animal draught cultivation system, which must then be brought together, modified and adjusted to achieve a workable relationship. Their interactions have not been examined as have those of tractors and implements, for which the attachment linkage (equivalent to the harness in animal draught systems) has been analysed in depth to ensure that the compromises that are always necessary in the design process have been made to the best possible effect. This is somewhat surprising as the basic analysis for animal draught systems is quite straightforward and has significant practical implications even in its simplest form. Field-testing is necessary to evaluate existing practices and could lead to their improvement through the introduction of new concepts.
The tillage implement draught equation and its importance in matching implement draught to the draught capability of work animals
The force system acting on and between a work animal and a tillage implement is illustrated and defined in Figure 3. Analysis of the interaction between forces (Inns, 1990) gives rise to the tillage implement draught equation, which states that:
H=V/tan a
Implement draught H increases with an increase of the effective vertical force V and decreases with an increase in the angle of pull a , and vice versa. The draught equation is therefore of decisive importance in guiding the design of integrated implement and harnessing systems to ensure that the implement's draught matches the draught capability of available animals.
The effective vertical force (EVF) is the net force acting downwards on the cultivation implement, the weight of the implement nearly always being the major and fundamentally inescapable factor. Forces from the soil will usually increase the EVF, although worn shares or incorrectly set points may cause a reduction until the implement suffers from loss of penetration, i.e. the EVF is reduced to zero. If the implement is excessively heavy, it may be fitted with wheels, skids or other devices to generate a support force from the soil when the implement has reached a particular set depth. This method of control was widely used in the past, but it is cumbersome, expensive and unnecessary if implements and harnesses are correctly designed and constructed.
The angle of pull a may be adjusted by changing the point of attachment of the pull chain or rope at either the implement end or the animal (harness) end. There is greater opportunity for adjustment at the harness end, given a suitable design, as the range of movement is potentially greater. Adjustment at the implement end is best used to "balance" the forces acting on it so as to ensure that the line of pull passes through the implement's "centre of resistance". Correct adjustment will ensure that the implement will then run level of its own accord without the need for a nosewheel or skid, facilitating any corrections for irregularities and reducing the operator's workload. A systematic design procedure will also guarantee that the range of hitch points on the implement are correctly located for this purpose.
Field trials in Scotland
The proposition that implement draught decreases as the angle of pull increases was investigated in field experiments at the Scottish Centre of Agricultural Engineering, Bush Estate, Edinburgh, in August 1994. The implement used was a Project Equipment EC1 light plough fitted with a 150-mm body from which the nosewheel had been removed, giving a total weight of 17.8 kg. The plough was pulled by a single donkey weighing 200 kg and fitted with a conventional breastband harness as shown in Figure 4. The harness was modified by the addition of a padded hip strap that could be adjusted in length to vary the angle of pull a, as in Figure 4. The draught force of the plough H was found by measuring the force P in the pull line and the angle of pull a, the draught is then given by H = P cos a. P was measured using a Novatech load cell of 250 kgf capacity with digital read-out, as shown in Figure 5.
A typical set of readings is shown in Figure 6. Each point is the average of about 50 readings with standard deviations as indicated. The angle of pull was about 20° without the hip strap, with a corresponding implement draught of about 70 kgf. The angle of pull could be raised to 35° or so by using the hip strap, at which angle the implement draught was reduced to 48 kgf. The draught reduction was accompanied by a modest increase in the vertical load carried by the donkey from about 26 kgf on the shoulders without using the hip strap to about 34 kgf on the hips when the hip strap was in use.
When adjusted correctly, the plough was stable in operation and easy to control without the need for a nosewheel. The steep angle of pull made it easy to raise the plough out of work when turning at the end of the furrow. It was obvious, however, that the donkey would not be able to work with the plough for a full working day. When pulling directly from the breastband, using the conventional breastband harness, the 70 kgf draught could be sustained by the donkey for only a few metres at a time. With the addition of the hip strap, on the other hand, the draught could be reduced to 48 kgf, but it could only be exerted with apparent comfort for short working spells and was excessive for continuous working by a single donkey.
The EC1 plough with a 150-mm body and pulled at an angle of 25° to 30° would be well matched to the draught capability of a single ox fitted with a suitable harness, such as a split-collar type, with an adjustable hip strap.
Field trials in the United Republic of Tanzania
The basic theory and results from the field experiments outlined above were used to design a new plough to match the draught capability of a single donkey, taken as heir between 20 and 25 kgf over a working day, based on assumed body mass of between 175 and 200 kg. The combination of theory and previous field trials suggested that to meet the draught target the plough must be as light as possible and it must be designed to work with an angle of pull up to about 35°. A working width of 115 mm was chosen for the plough body to ensure a reasonable depth work at this draught.
The single-donkey plough, shown in Figure 7, was built by a Tanzanian engineering company and weight 12.2 kg. Preliminary tests were made in Scotland using the same donkey as in the previous trials. Results confirmed that the plough could be adjusted so that it was stable in work and easy to control over the required range of pull angles without the need for a nose wheel or skid. The draught was responsive to variations in the angle of pull and the target range was met at higher pull angles.
More comprehensive field trials and farmer evaluations were arranged in cooperation with the Ministry of Agriculture Research and Training Institute (MARTI), Mbeya, Tanzania, benefiting from. the institute's extensive local contacts with small-scale farmers using draught animal power. Fields were chosen by the farmers from those in need of cultivation so that they represented a range of local conditions.
The variation of implement draught with angle of pull was assessed as before, with the results shown in Figure 6. At a pull angle of 20° the corresponding draught was 4 kgf, decreasing to 12 kgf at a pull angle of 35°. This enable the target range of draught (20 to 25 kgf) to be achieve' at pull angles between 27° and 30°, using the harness' adjustable hip strap. At these angles, a vertical load of about 12 kgf is carried on the animal's hips.
The farmers visited were well acquainted with ox cultivation. They kept donkeys for use as pack animals but were not accustomed to using them as draught animals The harness used was borrowed from MARTI and had hip strap added, as shown in Figure 7 The farmers (and their visiting neighbours) were initially sceptical of the work potential of such a small and light plough, but at the end of some hours work, during which it was tried by; number of people, they were satisfied (and somewhat surprised) with the quality and quantity of its output. They had no problems handling the plough without a depth wheel or skid but tended to control it with force rather than finesse, probably as a result of their experiences with heavier and more demanding equipment. An aggressive approach is not necessary, as was demonstrated by female operator who controlled the plough with gently exercised precision.
Further farmer evaluation is desirable. It is hoped that a number of single-donkey ploughs will be manufactured in Mbeya for evaluation and sale locally and in neighbouring districts, backed up by support services including training supply of replacement parts and attention to matters raised in feedback from farmers.
The work described above used the tillage implement draught equation to design an integrated animal, harness and implement system, probably for the first time. Nevertheless, the underlying relationships have always existed and have been implicit in developments made by innovative practitioners. For example, an early reference to the effect of steeper angles of pull on implement draught occurred in Pusey's article "Experimental enquiry on draught in ploughing?" (1840).
Implement
Excessive implement draught leads to premature fatigue of the work animals. Farmers often complain that available implements are "too heavy" for use with their animals - a complaint that is rejected by many manufacturers and advisers. The equation supports the farmers' intuitive understanding. Implement weight is a major proportion of the EVF, and, therefore, is a main contributor to excessive draught in most implements, which are indeed "too heavy".
It is sometimes claimed that weight is needed to enable the implement to penetrate to its working depth. Examination of the implement will usually reveal that the true reasons for poor penetration are worn or incorrectly aligned shares, tines or other components. Bad design may play a part. For example, the shape and/or setting of the mould-boards on some ridgers encourage excessive support from the soil, and, as a consequence, the EVF and the corresponding draught and depth of work may be reduced nearly to zero. Poor penetration is rarely a result of lack of weight.
An implement that is too heavy must be supported in some way to reduce its natural draught to an acceptable level. Wheels or skids add even more weight and their positioning and adjustment can upset the balance of an implement in ways that are not easy to understand and correct. The rational answer is to make the lightest implement consistent with adequate strength and with sufficient, but not excessive, draught to match the animal's capability. A lighter implement should be cheaper to make and easier to transport to and from the field.
Angle of pull and implement draught
Some researchers assert that, for greatest efficiency, the angle of pull should be as close as possible to the horizontal, arguing that a steeper angle of pull exerts a greater downward load on the animal, which is uncomfortable and tiring to support. This argument disregards the fact that the downward load is caused not only by the angle of the pull, but also by the magnitude of the load. A steeper angle of pull results in a lower implement draught and therefore a lower pull force required by the implement. The two effects are self-compensating in theory (for a constant EVF) but acceptably slight variations were noted in practice, as the two effects did not always balance exactly.
It may be difficult for some individuals to adapt to the expressed relationship between angle of pull and implement draught and to the profound influences on implement design, testing, adjustment and operation that result. The validity of the relationship can easily be tried in practice, however, and its full implications are worthy of exploration.
Conventional harnessing systems for bovines and equines, whether single or in pairs, result in a pull angle that is generally between 15° and 20°. For single animals most harnesses can be modified very easily, as shown in Figure 4, to increase the pull angle progressively up to 30° or more, enabling the implement draught to be reduced to about half its original value. It is not so easy to increase the pull angle for a pair of animals using a simple shoulder yoke - the pull chain may be shortened to increase the angle of pull, but only to a limited extent before the implement starts to foul the animals' hooves. For a pair of animals the best prospect for controlling implement draught is probably to use a well-designed beam-pulled implement (Inns, 1991).
If implement draught is reduced, so too is the work that the implement can do on the soil. There are no miracles to offset this fact. The primary objective of draught reduction is to alleviate the effort demanded from the animal - the alternatives are no work at all, because the animal will be incapable of significant movement against the too-high draught, or loss of work output, because the animal will be physically exhausted early in the working day.
The tillage implement draught equation has significant implications for the testing of animal-pulled cultivation implements. Because plough draught is affected by angle of pull and implement weight, these factors must be measured, recorded and taken into account whenever the performances of cultivation implements are tested and compared. The draught of different implements cannot be compared meaningfully unless this is done. Test procedures for animal-pulled cultivation implements must be revised or reformulated accordingly. These tests should include precise measurements to evaluate the quality of work output, such as width and depth of furrow, total area covered and total volume of soil ploughed effectively.
In the course of his investigations, the author has found evidence that many practitioners have been aware of the reduction of implement draught resulting from steeper angles of pull. Draught camels have a reputation for pulling a plough with ease, probably because the steep line of pull associated with these animals results in a reduction of draught compared with an equivalent plough pulled by other animals- an interesting factor to investigate. The saddle harness used with mules and horses in South America gives a steeper angle of pull than the collar harness and is associated with ease of work. Barwell and Ayre (1982) noted that the Japanese back harness, which is similar to the saddle harness but used with oxen, "... is more effective than the traditional neck yoke", as suggested by trials in India. Some locally made donkey harnesses incorporate a rearward attachment of the pulling traces, which gives a steeper angle of pull. Conversations with practitioners in the field indicated that some have found. from practical experience, that a steeper angle of pull is beneficial.
The relationship H = V/tan a shows that the draught of a cultivation implement is directly dependent on the vertical force acting on it (mainly its weight) and by the angle at which it is pulled. The work reported above shows how the equation may be used to design or adapt implements and harnesses so that draught requirements match the capability of the available work animal(s). It also has a profound influence on adjustments made to reduce implement draught and increase ease of use.
For ploughing, it is relatively easy to provide a matched outfit (plough plus harness) for single animals. The work capability of a single donkey was well matched by the 115-mm plough weighing 12.2 kg, which gave a draught of about 20 kgf when pulled at an angle of 30°. A single ox or horse with a body mass of approximately 500 kg would be well matched to the 150-mm plough weighing 18 kg, which gave a draught of about 54 kgf when pulled at 30°. The draught figures quoted relate to the particular field conditions in which trial runs were made - it may be necessary to change the pull angle to achieve the same draught under other conditions.
Implements must be adjusted specifically to allow higher angles of pull. This is not difficult or expensive, but expertise is necessary to position the attachment points correctly. The principles of adjustment are straightforward, logical and made easier by the absence of the nosewheel.
Any solution to an animal draught problem must be technically and economically viable and user-friendly in the context of local circumstances and infrastructure, otherwise, it is not a solution. Technical viability must be demonstrated in field use. Economic viability can only be assessed in the light of specific circumstances, but cost reduction consistent with acceptable performance will usually be beneficial. User-friendliness must be evaluated on the farm and in the field - it is a function of many factors including user interest, ease with which the equipment can be adjusted and used to optimum effect and satisfaction with the output and sustainability.
The work described in this article shows how the tillage implement draught equation, H = V/tan a, helps to achieve these criteria when matching tillage implements to draught animals The equation is also of value in solving a range of other problems +
Barwell, I. & Ayre, M. 1982. The harnessing of draught animals. London, UK, Intermediate Technology Publications. 92 pp.
Inns, F. 1990. The mechanics of animal draught cultivation implements. Part 1: Chain pulled implements. The Agric. Eng, 45(1): 13-17.
Inns, F. 1991. The mechanics of animal draught cultivation implements. Part 2: Beam pulled implements. The Agric. Eng., 46(1): 18-21.
Pusey, P. 1840. Experimental inquiry on draught in ploughing J.R. Agric. Soc. Eng., 1.
